Keep from freezing. The following nonprescription or herbal products may interact with sildenafil: cimetidine Tagamet HB , St. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. Many people have no side effects or only mild ones. If you are taking sildenafil for erectile dysfunction, your doctor will probably start you on an average dose of sildenafil and increase or decrease your dose depending on your response to the medication. The most common side effects are headaches, feeling sick, indigestion and dizziness.Viagra dosage for erectile dysfunction (ED) is usually milligrams (mg), but those over 65 may start at 25 mg. Your doctor will also need to consider possible drug interactions. Key facts. Sildenafil (Viagra) is a medicine used to treat erectile dysfunction (ED, also called impotence). Sildenafil works by helping to relax the blood vessels in the penis, allowing more blood to flow in and cause an erection. You should take sildenafil about an hour before planned sexual activity, so it has time to work. VIAGRA is available by prescription only in 25 mg, 50 mg, and mg tablets. Patients should always ask their doctors for medical advice about adverse events. You are .